There are known various systems utilized in electrographic printing. Generally, these systems use a pattern of charge which corresponds to a desired image. This charge is deposited upon photoconductive or dielectric surface of a drum or belt. This surface bearing the latent electrostatic image is moved through a toner station where a toning material of opposite charge adheres to the charged areas of the dielectric surface to form a visible image. The drum or belt is advanced forward and the toned image is either transferred to a receiving media or fused directly on the charged surface. After the fusing operation in the transfer system, the dielectric can be treated in various ways to clean its surface of residual charge or toner or both. This cleaning can be performed by any known electrostatic cleaning method.
In electrographic imaging and printing processes both photoconductive insulators and dielectrics have been used, however they are quite different from each other. Photoconductive insulators will only hold an electrical charge in the dark which makes them useful in limited applications such as copiers and the like. Dielectrics, on the other hand, can hold an electrical charge in the presence of visible light which makes them much more practical for use in commercial manufacturing processes such as the present invention.
There are also known many electrostatic printing systems such as those described in U.S. Pat. Nos. 3,023,731 (Schwertz); 3,701,996 (Perley); 4,155,093 (Fotland); 4,267,556 (Fotland); 4,494,129 (Gretchev); 4,518,468 (Fotland); 4,675,703 (Fotland); and 4,821,066 (Foote). All of these systems disclose non-impact printing systems using electrostatic images that can be made visible at one or multiple toning stations. In those systems ions are projected from an ion-generating means onto the surface of a dielectric layer by a print head such as described by Fotland in U.S. Pat. No. 4,155,093 or in U.S. Pat. No. 4,267,556. Generally, the print head comprises a structure of two electrodes separated by a solid dielectric member, a solid dielectric member and a third electrode for the extraction of ions. The first electrode is a driver electrode and the second is a control electrode; both are in contact with the separating dielectric layer. There is an air space at a junction of the control electrode and the solid dielectric member. A high voltage high frequency discharge is initiated between the two electrodes creating a pool of negative and positive ions in the air space adjoining the control electrode. The ions are extracted through a hole in the third electrode by an electrostatic field formed between the second and third electrodes. In Fotland U.S. Pat. No. 4,267,556 the image-forming ion generator takes the form of a multiplexed matrix of finger electrodes and selector bars separated by a solid dielectric member. Ions are generated at apertures in the finger electrodes at matrix crossover points and extracted to form an image on a receiving member. Grey scale control is achieved by pulse width modulation of the second (finger) electrode as described in Weiner U.S. Pat. No. 4,941,313. While prior art ion projection heads are useful in many applications, they are not adapted for use in systems requiring a relatively thick and hence low capacitance dielectric imaging layer. Generally, systems using ion projection printing technology utilize powder toners. In electrography, liquid development systems are best suited to accurate rendition of grey scale images and high resolution development. The components of toner systems can contaminate the electrodes in prior art ion projection heads and can render them substantially non-functional. When liquid toners are used, contamination of the ion projection cartridge is more of a problem than it is when using traditional dry powder toners. This is because the toner particles are considerably smaller in liquid toners than dry powder toners (e.q. 1 micrometer vs 25 micrometers) and also because there is a liquid component which evaporates. Thus, there is a high likelihood that the residual toner and/or solvents will migrate to the ion projection cartridge causing a loss of ion emission efficiency or total loss of emission. Incorporation of an air knife prior to the ion projection head can reduce the exposure of the head to contamination. The air knife will prevent exposure of the ion projection head to the toner particles and solvents in liquid toners by purging the space around the ion projection head with solvent free air or other gas. In addition, prior art projection heads are not particularly desirable for grey scale printing. Improved and novel ion projection heads are required to provide improved results in systems using liquid development systems and for those striving for acceptable grey scale density. Prior art ion projection heads are not only not particularly desirable for grey scale printing, but have substantial limits concerning the number of grey scales that can be achieved. For example, most can manage only to achieve 4 grey scales.
In addition to the deficiencies in prior art print heads, the known ion projection printing systems are not specifically designed to accommodate multicolored printing systems at rapid speeds. Therefore, while ion-generating systems utilize inherently sound technology there are several major improvements that need to be found before these systems can be used to produce multicolored final products of high print quality and at rapid speeds.